1. Field of the Invention
The present invention concerns an image forming apparatus for forming images by depositing a recording solution to an image support and an image forming method.
2. Related Art
Various proposals have been made so far for an image forming apparatus of a so-called ink jet system of applying an energy to a recording solution thereby forming and flying minute liquid droplets and depositing the droplets on an image support. The image forming apparatus of the ink jet system includes a scanning type using one or a plurality of nozzles on every color and scanning an image support to conduct recording or an array type using a plurality of nozzles arranged as an array. A mechanism of flying a recording solution used in the ink jet system is, for example, adapted to exert an external force such as pulse pressure on a recording solution in a cavity thereby jetting out particles of the recording solution from a nozzle. The pulse pressure is formed by pressure of air bubbles due to deformation of a piezoelectric transducer or heating and boiling of a recording solution.
In the existent ink jet system described above, since the recording solution has to be caused to fly by the pressure of air bubbles due to the heating of the recording solution or the deformation of the piezoelectric transducer, a recording solution of low viscosity at about 1-3 mPaxc2x7s is used. However, this involves the following basic problems. Namely, when recording is applied to common paper or like other paper sheet by using the recording solution at such a low viscosity, feathering occurs in which the recording solution blots a paper sheet along fibers of paper when the recording solution hits on the paper sheet, or the shape of dots to be printed is larger compared with a nozzle diameter and made uneven. Further, when recording is applied by using recording solutions of different colors, color mixing occurs to result in bleeding upon contact of dots adjacent to each other.
An approach for solving such problems has been disclosed, for example, in Japanese Published Unexamined Patent Application No. Sho 64-63185. FIG. 13 is an explanatory view of an existent recording method in one example of an image forming apparatus of an ink jet system for reducing blot. In the drawing are shown a pre-depositing solution head 41, a recording solution head 42, an image support 43, a pre-depositing recording droplet 44, and a recording droplet 45. At first, as shown in FIG. 13(A), a pre-depositing droplet 44 is caused to fly by using a pre-depositing solution head 41 to hit on the image support 43. The pre-depositing solution is a solution for insolubilizing the recording solution. In FIG. 13(B), a recording droplet 45 is caused to fly from the recording solution head 42 and the recording solution hits on the previously hit pre-depositing solution. Thus, both of the solutions are mixed as shown in FIG. 13(C) and the recording solution is insolubilized to be prevented from blotting the inside of the paper sheet. However, the blot preventing is still insufficient even in this method, as well as the pre-depositing solution used for the insolubilization of the recording solution is hit previously, so that secondary trouble is caused that the paper sheet itself is creased due to the pre-depositing solution.
The foregoing is problems resulting, particularly, in a case of using a recording solution of low viscosity. The problems may be solved basically by the use of a recording solution increased with viscosity. However, in the ink jet system of flying the recording solution by the pressure of air bubbles due to the deformation of a piezoelectric transducer or heating of the recording solution as described above, it was impossible to fly the recording solution of high viscosity or it required a great amount of energy for flying the droplets. Therefore, a system different from such an ink jet system has been developed.
In one of typical systems, a recording solution is formulated into special hot melt type. In this system, a recording solution which is solid at a normal temperature is used, and the viscosity of the recording solution near the nozzle of a recording head is usually lowered by heating, and a recording solution is jetted in a stringing manner. However, the viscosity of the recording solution usable for this system is limited to about 10 mPaxc2x7s. Further, there is also a problem of requiring heat energy for always heating the recording solution.
Another method capable of using a recording solution of high viscosity is disclosed, for example, in Japanese Published Unexamined Patent Application No. Hei 5-8384, in which a plurality of individual curved ink channels are formed as an array each at a predetermined interval in the inside of a polarizing body comprising a piezoelectric material, and individual electrodes and a common electrode are formed to partition walls of the individual ink channels and a voltage is applied to the electrodes, thereby causing a large displacement to the partition walls in the direction of the array. This increases a volume of adjacent individual ink channels in which the recording solution is filled and, subsequently, the recording solution in the recording solution chamber of increased volume is jetted out by the reaction caused upon returning of the displaced recording solution walls. The literature describes that the ink of high viscosity can be used but gives no detailed descriptions. Based on the content disclosed in the literature, it is considered that even if the displacement caused by the piezoelectric member is greatly increased by a curved shape, vibrations due to the resultant displacement are absorbed into the recording solution per se at a viscosity higher than 10 mPaxc2x7s and it is impossible to jet the solution. In addition, since the volume of the adjacent individual ink channels is also changed by the displacement of the partition walls, the amount of the recording solution is changed due to the change of the volume or the hitting position of the droplet is scattered to possibly deteriorate the quality of recorded images.
A further method capable of using a recording solution of high viscosity includes a method of printing a recording solution of high viscosity by depositing a recording solution to an image support without flying as described, for example, in Japanese Published Unexamined Application No. Hei 4-257485. FIG. 14 is a schematic cross sectional view illustrating an example of an existent image forming apparatus adapted to deposit a recording solution. In the drawing are shown a recording head 51, an ink chamber 52, an electric field application electrode 53, and an image support 54. The recording head 51 has the ink chamber 52 for storing an ink and a discharge port formed at a portion in contact with the image support 54 for communication with the ink chamber 52. An electric field application electrode 53 is disposed to the discharge port. An electric field is applied depending on an image signal to the electric field application electrode 53 to control the amount of the recording solution discharged from the discharge port. The usable viscosity of the ink ranges from 50 to 1000 mPaxc2x7s.
FIG. 15 is a view for explaining a disadvantage in one example of the existent image forming apparatus adapted to deposit a recording solution during recording. In FIG. 15 (A), an electric field applied to the electric field application electrode 53 is controlled to deposit the ink on the image support 54. Subsequently, when the image support 54 or the recording head 51 is moved relatively for forming a succeeding picture element, the ink cannot be discontinued merely by shearing force accompanying the relative movement but it inevitably causes a stringing state of ink as shown in FIG. 15(B). Further, since the recording head 51 is in direct contact with the image support 54, there is a problem that the ink deposited on the image support 54 is frictionally rubbed by the top end of the discharge port. In such a case, even if an ink of high viscosity is used, the image quality cannot be improved. Further, since only the ink having an electric viscosity can be used, the material of the ink is limited to an extremely narrow range. Furthermore, since the flow of the ink is controlled by an electric viscosity effect, it involves a fundamental problem that the discharged amount of the ink cannot be controlled if a power source for the apparatus main body is discontinued to possibly leak the ink from the discharge port.
The present invention has been achieved in view of the foregoing situations and it is an object thereof to provide an image forming apparatus and an image forming method capable of using a recording solution of a viscosity within a wide range, capable of forming a recording dot at high accuracy with a low energy and capable of obtaining high quality images of recording dots with reduced blot at high speed.
The foregoing object of the present invention can be attained by an image forming apparatus comprising a recording solution chamber for possessing a recording solution, a pressurizing unit for pressurizing a recording solution in the recording solution chamber, an extrusion opening disposed to the recording solution chamber, a shutter disposed to the extrusion opening and a shutter driving device for driving to open and close the shutter in accordance with image information.
In the present invention, recording is conducted by extruding a recording solution from an extrusion opening and depositing the solution on an image support. A shutter is disposed to the extrusion opening and the extrusion opening is opened only upon recording a dot. Further, after extrusion of the recording solution, the shutter is closed to stop the extrusion of the recording solution. Control for the extrusion amount of the recording solution is enabled by the provision of the shutter to the extrusion opening and drying up of the recording solution can be prevented by closing the extrusion opening with the shutter in a period of time not requiring discharge of the recording solution during printing or in a period of time in which the image forming apparatus is caused to stand-by. In the present invention, since the recording solution is not caused to fly but extrude and flow out continuously, the energy required for recording can be reduced even for a recording solution of high viscosity compared with a case of flying recording droplets. Therefore, it is possible to extend the range for the usable viscosity of the recording solution and form images with a low energy. Particularly, since the recording solution of high viscosity can be used, blot of the recording dot can be prevented, thereby capable of preventing feathering or bleeding.
Further, in a case of using a recording solution of high viscosity, the recording solution extruded out of the extrusion opening is continuous. Therefore, if the image support, for example, is relatively moved as it is, the recording solution causes stringing to form a long tail to the dot recorded on the image support. In the present invention, however, since the stringing of the recording solution can be discontinued by closing the shutter, degradation of the image quality caused by stringing can be prevented.
Furthermore, since the recording solution is pressurized, the time from the opening of the shutter to the extrusion of the recording solution through the extrusion opening and deposition of the recording solution on the image support is extremely shortened to enable high speed recording.